Does it really matter? These days decoding image with JS or Wasm should be fine. It's not video.
A mammogram on the other hand, might be 300 images of lossless 4k resolution, which could clock in at about 2 gigabytes. That could be per breast in a given study, and a study could have prior mammograms attached as well.
You will hit memory limits, so you need to be able to unload and load data intelligently and quickly.
Especially handy that it wouldn't require a new file format. It only requires the encoder to support variable compression. (I know Photoshop supported this for JPEG many decades ago.)
Foveated compression sounds like a super cool idea I've never thought about before.
Before that, you would need to get radiologists onboard with the idea of something being "diagnostically lossless" (think visually lossless), which is currently a hard sell (some promising research does exist on this)!